Depending on the reference, pin track infection rates in external fixation surgery have been stated to be anywhere from 0% to 100%. We critically evaluated the pin track infection rate for external fixation by performing systematic review of the external fixation literature since 1980. Using PubMed, a search of the peer-reviewed literature on external fixation was performed. This systematic review was conducted, as much as possible, in accordance with PICOS and PRISMA guidelines. A total of 150 articles were reviewed, including at least one from each year between 1980 and 2014. The following data were collected from each article: the year of publication, number of patients in the study, average age of the patients, reason for the external fixation, fixation per segment (two or more than two points), body part involved, whether or not hydroxyapatite-coated pins were used, duration of the external fixator, type of fixator used, and number of patients with documented pin track infections. These 150 studies represented 6130 patients. There were 1684 reported pin track infections from these 6130 patients, giving a cumulative pin track infection rate of 27.4%. A more recent year of publication was associated with an increasing infection rate (P = 0.015) while increasing age was associated with a decreased infection rate (P < 0.0005). There were trends toward association of humerus location (P = 0.059), shorter fixator duration (P = 0.056), and circular fixation (P = 0.079) with decreased infection rates. This systematic review of external fixation publications revealed a cumulative pin track infection rate of 27%. Younger age was the factor leading to increased pin track infection rates. Circular fixation trended toward being protective of pin track infection when usage was factored into the multiple regression analysis. Longer duration of fixation trended toward increased infection rate as expected. This data provides important base values for a common complication in external fixation treatment, highlights the importance of a more consistent definition of a pin track infection in future research, and identifies the pediatric population as the group at greatest risk.

The health and stability of the half pin to skin interface and the half pin to bone interface are critical in external fixation. Breakdown at either of these interfaces can result in a pin track infection. Despite innovations in half pin design and pin care regimens, pin track infections continue to be an anticipated nuisance of using external fixation. For such a common occurrence, however, the data on pin track infections in the external fixation literature remain limited, heterogeneous, and poor in quality. [1] Depending on the reference, pin track infection rates have been reported to be anywhere from 0% to 100%. [2],[3],[4],[5],[6],[7],[8] The vast discrepancy between these reported values makes this information difficult to interpret.

Creating a standardized scoring system for reporting pin track infections would help to make future studies on this topic more meaningful. The first step in this process is to attempt to identify a baseline pin track infection rate for external fixators. Once this starting point is established, further research could use this infection rate as a threshold for determining whether an intervention is successful or not. This study aims to critically evaluate the pin track infection rate for external fixation by performing a systematic review of the external fixation literature since 1980. By analyzing a large volume of pin track infection data from the literature, the goal is to define the baseline pin track infection rate for external fixation.

Materials and Methods

The PRISMA and PICOS guidelines were followed as much as possible during the conduct of this systematic review [Figure 1]. [9] Through PubMed database, a search using the phrase "external fixation" was performed. Under this heading, a total of 9620 articles were identified. The search limits "custom range of dates" (January 1, 1980, to June 30, 2014), "human only," and "English language only" were then created, and this decreased the total number of articles to 5493. The following exclusion criteria were then applied to the search list: Articles pertaining to external fixation of the axial skeleton (cranium/pelvis/spine) were eliminated; case reports were eliminated (a minimum of five patients per article); the results had to report pin track infections as the number of patients affected not by the individual pin or wire; the article had to be accessible via the online library of the primary author's institution. This process reduced the number of eligible studies to a final total of 150 that were then analyzed completely.

For each article reviewed, the following data points were extracted: Year of publication, number of patients in the study, average age, reason for the external fixation (trauma, lengthening, deformity correction, etc.), amount of fixation per segment (2 points or more than 2 points), body part involved (distal radius, femur, tibia, humerus, etc.), whether hydroxyapatite (HA)-coated pins were used or not, duration of the external fixator, type of fixator used (circular versus uniplanar design), number of patients with documented pin track infections.

A weighted multiple regression analysis was performed using SPSS Statistics Version 21 (IBM, Armonk, New York, USA). The analysis was weighted linearly based on the number of patients in each study. Pin track infection was the dependent variable. Independent variables included years since publication date; average age; treatment of trauma, deformity, or length discrepancy; location in the tibia, femur, lower extremity (hip, femur, knee, tibia, or foot), distal radius, humerus, upper extremity (shoulder, humerus, elbow, distal radius, or hand); presence of two or three pins per fixation unit, use of HA pins; and uniplanar or circular construct. Standard dummy coding was used for locations, pin number, and fixator construct. Studies that neither specify nor had a mixed group of any variable were assigned zeros in the dummy coding. Studies were excluded pairwise. In the multiple regression analysis, multicollinearity was assessed as negative based on variance inflation factor <10 and coefficient tolerance >0.1 and with intervariable correlations of lower than 0.7. The multiple regression analysis was repeated after removing any variable that violated multicollinearity. The lack of any undue influence from outliers was confirmed with a Cook's distance <1. Significant values were based on P < 0.05. Deformity as an indication, upper extremity location, and uniplanar construct was excluded from the analysis due to multicollinearity. Standardized beta values represent the relative contribution of each independent variable to pin track infection. For example, for age, –0.344 squared is 0.118 indicating that 11.8% of total pin track infection rate variance is explained by patient age. Unstandardized beta values represent the actual contribution of each variable to pin track infection. For example, for years from publication, –0.005 can be multiplied by 10 years to produce –0.05, indicating that one would expect 5% lower rate of pin track infection if a study was published 10 years earlier.

The pin track infection rate for pediatric patients (under age 18) was 38% (717/1893). The pin track infection rate for adult patients (age 18 and over) was 24% (896/3800). Among the adult patients, the 18-39-year-old age group had an infection rate of 27% (556/2082), the 40-64-year-old age group had 23% infection rate (309/1329), and the 65 and over years old age group had 8% infection rate (31/389).

The three most common etiologies for frame usage were trauma, deformity correction, and lengthening. Trauma patients had a pin track infection rate of 24% (986/4161). Deformity correction patients had a pin track infection rate of 29% (348/1199). Limb lengthening patients had a pin track infection rate of 46% (239/512).

The duration of time the frame was on the patient was analyzed. For a fixator removed in 42 days or less, the pin track infection rate was 19.6% (191/972). For frames with duration between 43 and 90 days, the pin track infection rate was 24.2% (475/1956). For frames removed between 91 and 150 days after application, the pin track infection rate was 27.2% (498/1828). For frames removed after >150 days, the pin track infection rate was 37.8% (440/1161). For frames removed over >180 days, the pin track infection rate was 47.8% (335/700).

Circular external fixators had 29.5% (457/1545) pin track infection rate compared to 22.9% (937/4089) pin track infection rate in uniplanar fixators. Details comparing the etiology of frame use and duration of frame use for circular and uniplanar fixators are provided in [Table 2].

Pin track infections were evaluated by decade. The pin track infection rate in the 1980s was 23.2% (301/1295). The pin track infection in the 1990s was 25.9% (424/1631). The pin track infection rate in the 2000s was 36.1% (573/1583). The pin track infection rate from 2011 to 2014 was 23.8% (386/1621). Details of the frame usage by decade are provided in [Table 3].

The stability of the fixation was analyzed. Many of the trauma articles, especially those involving pediatric and upper extremity fractures, only used two half pins in each bone segment. Patients with only two points of fixation per segment had 21.5% (566/2629) infection rate compared to 30.5% (651/2131) infection rate in patients with three or more points of fixation per segment. Details on the number of points of fixation per segment by patient etiology and frame duration are provided in [Table 4].

The pin track infection rate appears to be influenced, however, by multiple variables. In the regression analysis, standardized beta indicates the magnitude of each variable's effect on pin track infections. The variable with the greatest effect was patient age followed by publication year, circular fixation, and duration in decreasing order. Patient age and publication year had statistically significant effect (P < 0.05) while circular fixation and duration were nearly significant.

Examination of the patient's age demonstrates that pediatric patients have the highest risk of infection at 38%. Patients aged 65 and over had 4.75 times less risk of developing a pin track infection (8%). This information seems counterintuitive given that pediatric patients generally have competent immune systems without additional medical comorbidities. It is possible that adult patients are more compliant with the pin care regimen than pediatric patients. Unlike the adult patients who care for themselves, many pediatric patients rely on others to perform the pin care. This may lead to less regular care, especially from parents who are concerned about hurting their children while performing pin care or who simply do not have time to perform it on a consistent basis. Pediatric patients also had their frames on longer than patients aged 65 and older. Pediatric patients had the frame removed in six weeks or less 5% of the time (92/1893) compared to 38% (148/389) in aged 65 and older patients. The frame duration was 180 days or longer in 20% of the pediatric patients (377/1893) compared to only 2% in the 65 and over group (7/389). Another possible explanation is that pediatric patients have used frames for more complicated procedures than elderly patients. No frames for deformity correction or lengthening were used in the elderly population while 39% of the pediatric cases (736/1893) were deformity corrections and 21% were limb lengthening cases (397/1893). The increased complexity of the cases combined with the increased duration of the frame was probably influenced the difference in the age group pin track infection rate. Finally, pediatric patients are often quite active in their frames which can lead to increased friction at the pin-site interface as well as additional external contamination.

The pin track infection rate appears to have been getting worse over time despite improvements in technology and experience with statistical significance in the multiple regression analysis. The 1980s had the lowest rate of infection of all time periods (23.2%), and the pin track infection rate increased each decade from the 1980s to the first decade of the 21 st century (1990s = 25.9%, 2000s = 36.1%). The data from the last 5 years, however, show a recovery back toward the pin track infection rate of the 1980s (2010-2014 = 23.8%). It is hard to explain the differences in each decade since there were over 1200 patients in each group. The first decade of the 21 st century had the highest number of limb lengthening cases and the least number of trauma cases. Thus, these trends may reflect the increased use of external fixation for more complicated cases with time. Changes in the definition of a pin track infection with time may have also influenced this trend. Finally, it is recognized that the year of publication does not necessarily reflect the date of treatment. A group of patients may have been collected over a long period and therefore do not fully represent the pin track infection of the year (or decade) of publication.

Circular external fixators had a higher pin track infection rate (29.5%) than the uniplanar fixators (22.9%). This most likely represents a difference in usage rather than a true difference in pin track infection. The circular frames were used for a higher percentage of limb lengthening (10% to 4%) and deformity correction cases (33% to 13%) than the uniplanar frames. The circular fixators, therefore, also had a longer average duration on each patient exposing the pins to potential infection for a greater period. Circular frames were used for longer than 180 days in 17.6% of patients (273/1545) compared to 6.9% of patients (276/3959) in uniplanar frames. In the multiple regression analysis, circular external fixation actually trended toward a protective effect against pin track infections supporting the difference in usage.

The duration of external fixation trended toward a large influence on pin track infection rate. There was a steady increase in pin track infection rate as the length of external fixation increased. For frames removed within six weeks, the pin track infection rate was only 19.6%. The pin track infection rate grew 2.4 times higher (47.8%) in patients with external fixation for over 180 days. This supports the idea that the pin-skin interface degrades over time. The longer the pins are exposed to bacteria the more likely they are to become infected.

The etiology of the frame use (trauma, deformity correction, limb lengthening) produced gradually increasing pin track infection rates. Trauma patients had the lowest pin infection risk at 24% while limb lengthening had the highest risk at 46%. Frames for deformity correction were in-between these two groups with an average pin track infection rate of 29%. The differing pin track infection rates may be related to the duration of fixation. The trauma external fixators were removed in an average of 88 days while the limb lengthening frames remained on each patient more than twice as long with an average of 198 days.

The location of the frame on the body seemed to affect the rate of pin track infection. The distal radius had the lowest rate of infection at 12%, and the tibia had the highest rate of infection at 33%. The humerus and femur were intermediate with pin track infection rates of 23% and 26%, respectively. In the multiple regression analysis, there was a trend toward lower pin track infections with a humeral location and no clear effect at the other locations. Intuitively, it would seem that the femur should have the highest pin track infection rate due to the large soft tissue envelope surrounding the half pins and wires in this location. However, the data did not support that theory. Lower extremity pins and wires were twice as likely to become infected (31%) compared to upper extremity pins and wires (14%). Two factors may help explain the discrepancy. First, the upper extremity frames tended to remain in place for shorter periods compared to the lower extremity frames. Second, the upper extremity frames were mainly used for trauma applications rather than more complicated deformity corrections or limb lengthening procedures. It is also possible that there are some inherently protective effects in the upper extremity that allows for a decreased pin track infection risk. The upper extremity usually has a smaller soft tissue envelope than the lower extremity, and the upper extremity is more vascularized than the lower extremity.

The use of HA-coated pins did not seem to show a protective effect to the patient with regards to the pin track infection rate. While the sample size of literature using HA-coated pins is relatively small (240 patients), their pin track infection rate was higher than the pins without the coating. The HA coating enables bone formation directly on its surface by creating a chemical bond between the crystals of the coating and those of the newly formed bone. [166] This has been shown to increase the removal torque for half pins, indicating improved stability of the pin-bone interface over time, especially in metaphyseal bone. [3],[167],[168] Despite its ability to improve the pin-bone interface, however, HA coatings have no direct antibacterial effect and are still vulnerable to bacterial infection like uncoated pins. Infections of external fixator pins are the result of bacterial adhesion followed by the development of a biofilm. [169] For a pin coating to effectively decrease the risk of pin track infection, it would need to contain an active antimicrobial agent that lasts for the duration of pin implantation. This concept has been attempted unsuccessfully in the past with the use of silver. [170] Iodine-supported titanium half pins, however, have recently been developed and represent a promising potential advancement in the prevention of pin track infection. [171]

This study has several limitations. Most importantly, the definition of a pin track infection is not standardized, and the results reported in each individual article vary depending on the method used to diagnose the infection. The literature on pin track infections is therefore difficult to interpret, and published rates of pin track infection vary widely, from 0% to 100% depending on the source of the information. [2],[3],[4],[5],[6],[7],[8] There are many reasons for this considerable discrepancy, and several classification schemes have been proposed with different criteria for identifying a pin track infection. [172],[173],[174],[175],[176] In addition, some authors choose to report the pin track infection rate as the number of patients who develop an infection over the course of treatment. This is the most common method of reporting but it has potential flaws. This reporting technique does not indicate if the patient has more than one pin or wire infected and it does not indicate if the same pin became infected more than once during the course of treatment. The alternate method of reporting infections is to calculate the pin track infection rate as the number of pin track infection occurrences divided by the total number of pins or wires. This technique provides more data but it is more difficult and time consuming to monitor. It also does not indicate if the same pin or wire has been infected more than once. Finally, pin track infections are the result of multiple variables. The pin location, the insertion technique used, the patient age, the type of external fixation used, the etiology of the external fixation, the duration of the external fixation, and the presence of any protective coating on the pin are some of the factors that can influence whether or not a pin track becomes infected. This information is typically consolidated in the external fixation literature, making it difficult to get a true sense of the pin track infection rate.

In summary, this systematic review of pin track infections associated with external; fixation revealed a cumulative rate of 27%. Younger age was the largest factor leading to increased pin track infection rates. Circular fixation trended toward being protective of pin track infection when this was factored into the multiple regression analysis. Longer duration of fixation trended toward increased infection rate as expected. Later publication year was associated with increased infection rate, possibly related to either changes in use or changes in the definition of a pin track infection over time. These data provide important base values for a common complication in external fixation treatment, highlight the importance of a more consistent definition of a pin track infection in future research, and identify the pediatric population as a the group at greatest risk. Developing a standardized scoring system for each pin or wire site may help to eliminate some of the many variables that have been the source of confusion in existing pin track infection literature.

Conclusion

This systematic review of external fixation publications revealed a cumulative pin track infection rate of 27%. Younger age was the factor leading to increased pin track infection rates. Circular fixation trended toward being protective of pin track infection when usage was factored into the multiple regression analysis. Longer duration of fixation trended toward increased infection rate as expected. This data provides important base values for a common complication in external fixation treatment, highlight the importance of a more consistent definition of a pin track infection in future research, and identify the pediatric population as the group at greatest risk.

Acknowledgements

We would like to thank Jordan Grauer for assistance in the preparation of this manuscript.

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